Learn About the Technologies

Learn all about the technologies that power projectors, rear projection televisions and plasma displays with our easy-to-understand explanations and diagrams.

LCD

We’ve already learned how a flat panel LCD works in the section Learn about LCD Displays. Now it’s time to learn how the technology works in a projector.

An LCD optical light engine incorporates three small LCD panels (see Figure 1.): a lamp, filters and a prism to create an image.

White light is provided by way of a lamp. The lamp light is first passed through a polarizing filter, and is then passed through a series of dichroic mirrors (a mirror that reflects only certain wavelengths of light while letting all others pass through), which split the white light into its primary colors: red, green and blue. The three primary colors are then passed through three separate LCD panels and sent to a dichroic prism, which recombines the three primary colors. After the three colors are recombined, the light is passed through the main lens and out of the projector. See Figure 2.

Each LCD panel controls one of the three primary colors. For example, if we were to show an image of an open green field with a blue sky, the LCD panel that controls red would block all light from passing on to the dichroic prism, while the LCD panel that controls blue would only allow blue light through in the of sky. Meanwhile, the LCD panel for green would do the same for the areas of the green field.

How does the LCD panel itself work? The light is polarized first. Think of the polarization process as a filter that only accepts light waves that are all traveling on the same plane. In this way, if you were to put two polarized filters back-to-back that filtered the light in opposite planes, all the light would be blocked from passing through.

This polarized light travels to the LCD panel and passes through a pane of glass to the liquid crystal inside. Liquid crystals have the unique property of bending light as the light travels though. In this manner, the polarized light leaves the liquid crystal, actually traveling on a different plane that when it entered.

If you apply current to liquid crystal, the crystals align themselves and will not bend the light. Instead the light leaves the liquid crystal on the same plane that it entered.

If you align a second polarizing filter after the light passes through the liquid crystal, you can block all light from escaping if the light waves are traveling on a plane that the filter blocks. See Figure 3.

Each LCD panel has individual cells that are each controlled by a separate transistor that can apply current to the liquid crystal within the cell. The resolution of the display determines the number of cells, or pixels, that the LCD panel has.

In this way, each panel can control what color each pixel is going to be at any given moment. If a pixel needs to be white, all three panels let through light at their corresponding pixel and the three primary colors are recombined by the dichroic prism to create white light.

All pixels working together in the projector can then paint a picture in much the same way a printer uses individual dots to make a color image on paper.

DLP™

How does a DLP™ projector work? Let’s start with the basics. First, like with all microdisplay technologies, they have a high output lamp that creates white light.

Now white light itself can’t create a full color picture, so there has to be a way of splitting that light into its three primary colors: red, green and blue. DLP™ light engines achieve this by passing the light through a spinning color wheel (see Figure 1). The color wheel is simply a red, green and blue filter that spins in front of the lamp to create the primary colors, one after another.

We now have red, green and blue light. This light is directed to a small silicon chip called a DMD™ device by Texas Instruments (see Figure 2).

The DMD ™ device is a small chip that consists of hundreds of thousands of tiny mirrors that can tilt in two directions, designated as on and off positions, at thousands of times per second. See Figure 3.

Each mirror represents one spot, or pixel, of the image. All of these pixels, when combined horizontally and vertically, make up the entire image. This is much in the same way as your printer makes a printed copy for you.

So how does it make an image? Let’s take that first bit of light that arrives at the DMD™ chip after it passes through the color wheel: red light. At the time the red light strikes the chip, all of the mirrors that correspond to where red should appear within the image will turn “on”. When the pixel is turned “on”, it directs the light out of the lens and to your screen. For the places that contain no red, or no color that is comprised of red, the mirror is turned “off” and the light is then directed away from the lens and to a light collection space within the projector. The DMD™ chip then does the same for the next two colors (blue and green, respectively) in sequence, thereby making up an entire image.

How does the DMD™ create shades of colors that are not pure red, green or blue, and how does it create an image if the red, green and blue are not shown at the same time?

It all has to do with timing. The DMD™ displays shades of red by varying the time it keeps red on the screen. The shorter the time on the screen, the less the eye is able to detect the color, resulting in a lighter shade. It creates colors and whites the same way. Displaying all three primary colors, one after another for the same time period, produces white. If you wish to display a shade of gray, then each color is produced for a shorter time period. To produce colors, the DMD™ will vary the amount of time each color is on the screen to produce different colors at different shades.

LCoS

(a.k.a D-ILA, SXRD and Reflective LCD)

Liquid crystal on silicone (LCoS) technology has a few variances that are in the market place; JVC calls its version D-ILA, while Sony has named its version SXRD. Nonetheless, all versions work in pretty much the same manner.

Think of LCoS technology as an LCD on a silicone wafer. To build on that, let’s start with how a liquid crystal in a LCoS panel works.

Polarized light is first sent to the LCoS panel. Think of the polarization process as a filter that only accepts light waves that are all traveling on the same plane. In this way, if you were to put two polarized filters back-to-back that filtered the light in opposite planes, all of the light would be blocked from passing through.

This polarized light travels to the LCoS panel and passes through a pane of glass and transparent electrode to the liquid crystal inside.

Liquid crystals have the unique property of bending light as the light travels though them. In this manner, the polarized light leaves the liquid crystal, actually traveling on an entirely different plane that when it entered. In the case of an LCoS panel, the light is reflected off the silicone layer behind the liquid crystal and back out of the front of the LCoS panel.

If you apply current to liquid crystal, the crystals align themselves and will not bend the light. Instead, the light leaves the liquid crystal on the same plane that it entered.

If you align a second polarizing filter to filter the light that was reflected through the LCoS panel, you can block all light from escaping if the light waves are traveling on a plane that the filter blocks. See Figure 1.

Now we can talk about how the panels are used in an optical engine to produce and image.

An LCoS optical light engine incorporates three small LCoS panels (AKA SXRD or D-ILA panels; see Figure 2.), a lamp, filters and prisms to create an image.

White light is provided by way of a lamp. The lamp light is first passed through a dichroic mirror (a mirror that reflects only certain wavelengths of light while letting all others pass through) that splits the white light into yellow and blue light.

The yellow light is then passed on to an additional dichroic mirror that splits the yellow light into red and green light.

The red, green and blue light is then passed through polarizing filters before being sent to directional prisms that reflect light coming from a certain direction, but allow the same light to pass through if coming from a different direction. These prisms are used to bounce the light off of the three LCoS panels.

After the red, green and blue light is bounced off the LCoS panels, it is sent to another dichroic prism, which recombines the three primary colors. After the three colors are recombined, the light is passed through the main lens and out of the projector. See Figure 3.

Each LCoS panel controls one of the three primary colors. For example, if we were to show an image of an open green field with a blue sky, the LCoS panel that controls red would block all light from passing on to the dichroic prism, while the LCoS panel that controls blue would only allow blue light through for the areas of sky. Meanwhile, the LCoS panel for green would do the same for the areas of the green field.

Each LCoS panel has individual cells that are each controlled by a separate transparent transistor that can apply current to the liquid crystal within the cell. The resolution of the display determines the number of cells, or pixels, that the LCoS panel has.

In this way, each panel can control what color each pixel is going to be at any given moment. If a pixel needs to be white, all three panels let through light at their corresponding pixel, and the three primary colors are recombined by the dichroic prism to create white light.

All pixels working together in the projector can then paint a picture in much the same way a printer uses individual dots to make a color image on paper.